936 



Popular Science Monthly 



oscillation may be determined by wave- 

 meter measurement, or may be computed 

 according to the rule given in the March 

 article. Speaking generally, what hap- 

 pens is that the dielectric of the 



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Fig. 3. Curve showing the rise and fall of 

 current by varying the frequency 



condenser is electrically strained in one 

 direction when the charging voltage (or 

 pressure) is applied to it ; as soon as 

 the pressure is relieved, the strain reacts 

 and its energy produces a current 

 through the circuit (Fig. i) from the 

 positive plate of the condenser toward 

 the negative side. In passing through 

 the inductance the current sets up a 

 strong magnetic field, which expands and 

 stretches away from the coil as the 

 current through it grows larger. Since 

 there was only a definite amount of 

 electrical energy forced into the conden- 

 ser by the original charging voltage, 

 there is a limit to the amount of current 

 which can be produced by the discharge; 

 as soon as this limit is reached the 

 magnetic field around the coil L begins 

 to contract, and adds its energy to the 

 current flowing toward the negative side 

 of the condenser. By this time the 

 condenser is fully discharged, that is, 

 the two plates are at the same potential. 

 But the magnetic field is still collapsing 

 on the coil, and therefore, current is 

 forced to continue flowing in the same 

 direction as before; this results in a 

 piling up of potentials on the "negative" 

 plate of the condenser and a reduction 

 of electrical pressure on the plate which 

 was "positive." In other words, the 

 reaction of the magnetic field has 



caused the condenser to assume a new 

 charge, of polarity opposite to that 

 which it had originally. The pressure of 

 this inverted charge increases until 

 the energy of the magnetic field is 

 exhausted; then the condenser dis- 

 charges once more, but in the opposite 

 direction. A current flows back through 

 the inductance, and an expanding field 

 is set up, just as before, except that the 

 polarity is reversed. The contraction 

 of this second magnetic field forces a 

 new charge upon the condenser, and 

 this time the polarity is the same as of 

 that which began the oscillation. Since 

 a limited amount of energy is set free 

 in the circuit, and since some of this 

 energy is used in heating the wires 

 (because of their resistance) each suc- 

 cessive charge and each successive 

 current is smaller than that which 

 preceded it, and the free oscillation is 

 damped, as shown in Fig. 2. The greater 

 the resistance of the circuit the greater 

 the proportion of the original energy, 

 which is lost in heat at each oscillation, 

 and the sooner the current is damped 

 down to a very small value. 



What has this internal action of a 

 resonating circuit got to do with its 

 resonant condition, or its tuning (which 

 is much the same thing)? In a word, 

 everything. Why? Because "tuning" 



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Ci/c/ej 



Fif 



4. Curve showing changes in current 

 by altering the resistance 



is little more than taking advantage of 

 this self-swinging power of a circuit so 

 that energy may be added to it at just 

 the right time to give its oscillations the 

 largest amplitudes possible. In adding 

 small amounts of energy to an oscillating 

 electrical system, the addition must be 

 made by the application to it of corre- 

 sponding magnetic or electric forces. 

 That is, small charges must be put upon 



